1. Field of the Invention
This invention relates to an arm mechanism of a pushbutton tuner.
2. Description of the Prior Art
In a pushbutton tuner a desired frequency is selected by inserting a core into a tuning coil as far as a predetermined position, which core is moved together with a memory slide, which is pushed by a setting plate so that its displacement is varied by varying appropriately the mounting angle of the setting plate disposed on an arm of the tuner.
For such a pushbutton tuner, heretofore, a structure, as shown in FIG. 1, has been adopted in order to fix the setting plate by a desired angle to the arm. That is, the setting plate 3 is mounted rotatably on a surface of the arm 1 by using an axle 2, and rotational displacements of the setting plate 3 on the arm are prevented by pressing the other end of the setting plate downwards by a plate spring 4 assisted by a spring-compressing setting pin 5. However, since the setting plate 3 is pressed downwards by the plate spring 4, the setting pin 5 used as a fulcrum for the plate spring 4 should be high, and consequently such prior art type of pushbutton tuners have the disadvantage that the thickness of the whole arm is relatively great.
Accordingly, recently, in order to fulfil the requirement of making a pushbutton tuner thinner, instead of the prior art type shown in FIG. 1, it has been proposed to use an arm mechanism in which the setting plate is fixed by nipping it at its periphery (FIGS. 2A and 2B). That is, a setting plate 13 is mounted rotatably on a surface of the arm 11 by utilizing a setting pin 12 having a flange at its upper end. At the lower surface of this setting plate 13 is formed in one united body a boss 14, whose periphery is arc shaped. The periphery of this boss 14 is surrounded by an approximately U-shaped plate spring 15, and fastened by the plate spring 15 by pressing it from both the sides by means of a pushbutton so that the setting plate 13 cannot rotate around the setting pin 12. For such an arm mechanism the thickness of the whole arm mechanism can be small indeed with respect to that shown in FIG. 1, because the setting plate 13 is pressed by the plate spring 15 horizontally from both the sides and effectively serves to make the pushbutton tuner smaller and thinner. However, when this type of pushbutton tuner is adopted in practice, it gives rise to inconvenience. That is, since the setting plate 13 should be mounted rotatably with respect to the setting pin 12, there are always some play between the setting pin 12 and the side wall of the hole formed for it in the setting plate 13. Consequently, due to this play, it may be feared that the center position of the setting plate is different for the case where the setting plate rotates freely and for the case where it is fixed by the plate spring. When the center position of the setting plate is thus not at a well determined position, it is difficult to effect an accurate positioning of the setting plate according to the frequency to be selected. This gives rise in turn to errors in frequency, and the performance of the tuner is considerably lowered.